I guess it means making certain amount of gold equal to a certain amount of silver, rather than them being independant of each other.
Read this:
http://www.dani2989.com/gold/goldsirverratio180027092004gb.htm
It should help make things more clear.
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I think that initially silver dollar and gold dollar would be equivalent, eventually they would not due to difference in availability in the raw gold and silver around the world.
This may lead to confusion for there may be 2 prices for items. Overall it is better than the fiat system right now.
There may even be more different kind of dollars around due to different metals that could back them.
Of course, people can if they choose to, do all their transactions in just one type of currency to avoid confusion (since its legal tender).
The recommended ratio (non-enforceable of course) of one commodity money to another would be publicised by congress monetary committee daily (in the interest of a stable economy ie. no inflation or deflation).
The banks would certainly switch peoples notes and coins around for them into any type of commodity money they want, people could have separate gold, silver or other metal accounts, such as platinum at the bank.
The banks would have 100% backed account balances and if they wanted to loan money out to make extra profits they would be limited on drawing from a pool of term deposits (ie. those moneys the depositor pledges not to touch for a given amount of time in return for certain rate of interest). That way 100% backing is maintained.
So apart from earning the interest difference between loan and term deposit rates the other way the bank could make money is by levying account keeping fees as well as some other fees such as transaction fees or dishonour fees etc.
100% honest system, fair to the poor, middle and the rich.
Thats basically how I see this competing commodity money system working.